High temperature process for modifying thermoplastic filamentous material

Abstract

Filaments having a substantially increased inner surface, in particular porous filaments of filaments having a cracked, notched or nicked surface of thermoplsatic material are produced by subjecting filaments of thermoplastic material, preferably containing a pore-forming agent, to a heating medium at a temperature at least 100°, preferably 150°C. above the glass transition temperature of the thermoplastic material, preferably near or above its softening point, the temperature of the heating medium and the time of exposure of the filaments thereto being such that the peripheral portions of the filaments are affected by the heat treatment to a substantially larger degree than the core portions creating within the filaments temporarily a temperature gradient between the core and periphery of the filaments, the temperature at the periphery being sufficient to reduce the intermolecular cohesion of the thermoplastic material at the periphery, and the action of heat at the core being substantially lower than at the periphery.

What is claimed is: 
     
       1. Process for substantially increasing the total surface area of filaments of a thermoplastic polymer having an oriented crystalline structure by forming therein vacuities which are at least microscopically visible, and/or voids, indentations or cracks on the surfaces of said filaments which comprises: 1. contacting said filaments with a pore-forming or void-forming agent to incorporate therein from about 0.1 to about 20%, by weight, of said filaments, of said pre-forming or void-forming agent;   2. subjecting said filaments for a period of less than about 15 seconds and in the absence of longitudinal stress to a heating medium which is at a temperature of at least 100°C. above the glass transition temperature of said polymer, the temperature of said heating medium being such that the polymer would lose more than 85%, of its tensile strength if said filaments were subjected thereto for a period of more than about one minute, the temperature and time of exposure to said heating medium being such that the peripheral portions of the filaments are affected to a larger degree than the inner portions, thereby creating between the periphery and the inner portions a temperature gradient, and   3. immediately cooling said filaments by subjecting them to a cooling medium, said cooling reversing said temperature gradient, said pore-forming or void-forming agent comprising a liquid composition capable of being volatilized at least partially at the temperature to which said filaments are heated by said heating medium and being a non-solvent for said polymer but having a swelling action for said polymer at least at the temperature to which said filaments are heated by said heating medium and will not cause chemical degradation of the polymer to any substantial degree.   
     
     
       2. The process according to claim 1 in which said filaments are stretched to at least 5% of elongation at break. 
     
     
       3. The process according to claim 1 in which the voids formed in said filaments during said heat treatment are filled with a finishing agent by immersing the filaments while hot in a cooling liquid containing said finishing agent. 
     
     
       4. The process according to claim 1 in which said filaments are cooled to a temperature below room temperature prior to said heat treatment. 
     
     
       5. The process according to claim 1 in which said filaments are introduced to a cooling medium which is at a temperature below room temperature subsequent to said heat treatment. 
     
     
       6. The process according to claim 1 wherein said filaments are in the form of yarn, or woven, non-woven or knitted textile sheet material. 
     
     
       7. A process according to claim 1 in which said heating medium is at a temperature of at least about 150°C. above the glass transition temperature of said polymer. 
     
     
       8. A process according to claim 1 wherein said heating medium comprises gas or vapor. 
     
     
       9. The process according to claim 1 in which said heating medium comprises a fluidized bed of finely divided solid particles. 
     
     
       10. The process according to claim 1 in which said heating medium comprises a liquid heated to a temperature below its boiling point. 
     
     
       11. A process according to claim 1 in which said cooling medium comprises a gas or vapor. 
     
     
       12. The process according to claim 1 in which said pore-forming agent is a liquid having a boiling point below the temperature of the heating medium. 
     
     
       13. The process according to claim 12 in which said pore-forming agent is water. 
     
     
       14. The process according to claim 1 in which said filaments are subjected to longitudinal stress after being cooled. 
     
     
       15. The process according to claim 1 in which filaments after cooling are subjected to longitudinal stress such that the filaments are stretched to from 10 to 50% of the elongation at break. 
     
     
       16. The process according to claim 1 in which the heating medium is at a temperature within about 10°C. of the softening point of the polymer.